Following increase of a storage capacity and reduction of an access time of a rotation driving part of a magnetic recording apparatus, such as a hard disk driver (hereinafter referred to as "HDD"), for example, a high rotational speed and high rotational accuracy corresponding thereto are increasingly required to a driving spindle motor of the HDD in recent years. In order to rotate a precision motor to which such a high rotational speed and high rotational accuracy are required, employment of a gas bearing (dynamic pressure gas bearing) for the rotation driving part is proposed.
When a rotator rotates in the rotation driving part employing this dynamic pressure gas bearing, air is forcibly introduced at least into a clearance between a radial gas bearing body and the rotator. Hence, the air pressure in the clearance is increased, and the rotator rotates at a high speed through the dynamic pressure gas bearing. Thus, maintenance of the rotational accuracy is expected also during high-speed rotation, due to the employment of the dynamic pressure gas bearing.
In the aforementioned radial gas bearing, a wedge-shaped clearance is formed by eccentricity of a shaft body in the bearing body, as described in "Gas Bearing" by Shinichi Togo, Kyoritsu Shuppan (1989). Pressure is generated since air is compressed when the air passes through this wedge-shaped clearance. Thus, it is possible to support the shaft body and the bearing body in a noncontact manner.
A concrete structure of such a dynamic pressure gas bearing is described in Japanese Patent Publication No. 4-21844, for example. The structure described in this gazette is now described as a conventional dynamic pressure gas bearing structure.
FIG. 18 is a sectional view of a principal part of the conventional dynamic pressure gas bearing structure. Referring to FIG. 18, a shaft body 31 is arranged in a hollow cylinder of a bearing body 32. A sectional shape of this shaft body 31 in the radial direction has such a shape that a plurality of grooves 31b in the form of substantially L-shaped notches are equally distributed and provided on the outer periphery of a circular cylinder 31a having a section of a substantially complete round.
In this conventional dynamic pressure gas bearing structure, the shaft body 31 is so structured as to rotate with respect to the bearing body 32. When the shaft body 31 rotates, air is caught in the grooves 31b provided on the outer periphery of the shaft body 31, and dynamic pressure is generated by combination of vortex motion of the caught air and wedge action with respect to the inner peripheral surface of the bearing body 32. Due to this dynamic pressure, it comes to that the shaft body 31 and the bearing body 32 are supported in the radial direction in a noncontact manner in the rotational operation thereof.
In the conventional dynamic pressure gas bearing shown in FIG. 18, however, dynamic pressure cannot be efficiently generated between the shaft body 31 and the bearing body 32 in a low rotational speed area after rotating/starting the shaft body 31. Therefore, the shaft body 31 and the bearing body 32 cannot be shifted from a contact state to a noncontact state at a low rotational frequency. Thus, there has been such a problem that abrasion powder results from continuous contact between the shaft body 31 and the bearing body 32 up to a high rotational frequency shifting the same to the noncontact state, and galling is caused between the shaft body 31 and the bearing body 32 by the abrasion powder.